1. Field of the Invention
The invention relates to a permanent coating for metal equipment for the shaping of a mass of formable glass, especially glassmaking moulds, the coating being provided in order to prevent any sticking of the mass of glass to the surfaces with which it comes into contact.
In the quantity production of hollow glass articles, three principal moulding processes are used: blowing a parison in a mould, pressing a gob of glass, of predetermined size, between a die and a punch, and centrifugal casting. The last-mentioned process is used mainly in the manufacture of special articles; pressing is largely used in the manufacture of industrial articles, such as glass paving-blocks, table-ware and certain articles of hollow-ware; blow moulding is used almost exclusively for making bottles and flasks, and to a great extent for hollow-ware.
Whichever process is used, the external shape of the article (and equally its internal shape in the case of pressing) is determined by the shape of an internal cavity of a metal mould against the surfaces of which the glass, brought to a temperature at which its viscosity has an appropriate value, is pressed against the mould surfaces so as to assume their shape and become sufficiently cooled to be solidified in this shape, that is to say to have acquired a high enough viscosity not to suffer any significant deformation in the course of handling.
It will be recalled that the working level of a glass is the level of temperature at which the glass has a viscosity high enough for the glass to deform only slowly, but low enough to enable it to deform under the influence of moderate forces. In addition, all glasses possess a transformation point at which the expansion curve shows a knee and below which rapid cooling sets up internal stresses. It is generally accepted that the viscosity of glasses at the transformation point is 10.sup.14 Pa.s.
In quantity production, metal moulds are used which have sufficiently high thermal conductivity to solidify the glass rapidly by contact. This conductivity is all the more necessary as the moulds must be hot enough before moulding to avoid cooling of the glass when they make contact below the transformation point. Currently the metal most commonly used is cast iron, having a stable shape when hot, and being easy to work with fine polishes, and relatively less susceptible to sticking than the steels; some special steels (refractory non-oxidising) are also used; alloys of copper have been employed to facilitate cooling of the glass and maintenance of the mould temperature at high production rates. These alloys are being used less and less.
It goes without saying that it is necessary to prevent the glass from sticking to the walls of the mould, adhesion being liable to cause on the one hand superficial tearing of the article being moulded, and on the other hand poor stripping of the glass along the mould surfaces, with local restraint of its sliding movement. There is then a risk of the fault being repeated on articles to be moulded subsequently, if the adhered glass is not cleaned from the mould. These operations to restore the moulds lead to loss of production, and premature wearing out of the moulds due to the abrasive character of the glass.
It is known that adhesion of glass to a metal results from partial solution exchange between an oxide film formed on the surface of the metal, and constituent oxides of the glass (basically silica, lime and soda for current soda-calcium glasses). Adhesion varies greatly with temperature, which determines the growth of the oxide layer on the metal, and the rates of diffusion of the oxides one into another.
2. Description of the Prior Art
Conventionally, adhesion of the glass to the mould surfaces is prevented by coating the latter with a stripping agent.
The stripping agents most commonly in use consist of graphite suspensions with a binder, which is pulverised on to the mould surfaces. The binder itself adheres to only a limited extent, after the liquid in which it is suspended has evaporated and the mould has been heated to its working temperature; friction between the viscous glass and the mould surfaces results in local erosion of the graphite layer. It is therefore necessary to apply the graphite suspension afresh so as to restore the layer of stripping agent.
In certain cases, carbon black, obtained by sooty combustion of a hydrocarbon (propane, acetylene) is deposited on the mould surfaces.
It has been proposed to treat the internal cavity of the moulds with nickel so as to reduce the tendency to stick. This treatment is performed by displacing nickel with the iron of the mould in a solution of a nickel salt (chemically referred to as nickelising). Nickelising ought to reduce the tendency towards oxidation of the moulds. The resulting improvement has however proved to be insufficient to allow pulverisation of a stripping agent to be omitted.
The document U.S. Pat. No. 4,003,867 describes a coating composition based on pigment and aluminium phosphate. The coating layers are substantially thicker than the layers of carbon suspension, being in a range between 0.025 mm and 0.4 mm, and the coating is cured at between 105.degree. and 760.degree. C. It is to be noted that, when the coating is applied to the surfaces of a glassmaking mould, the pigment is powdered graphite, while for other applications, such as the coating of steel billets, the pigment may be of molybdenum bisulphide, mica, talc, powdered glass, or another lubricating pigment.
In a glassmaking mould, such a coating, of 0.15 mm, may last for 96 hours.
At the present time, it does not appear that the type of coating taught by U.S. Pat. No. 4,003,867 is in current use. It may be thought that, on the one hand, the surface condition of this covering is insufficient, and that, on the other hand, the presence of the covering renders the shape of the glass article imprecise and makes accurate closure of the mould difficult, since the presence of the coating surfaces in the plane of the joint of separation of the mould prevents a perfect fit.
Adhesion of glass to the equipment associated with glassmaking moulds, such as troughs and chutes for delivering viscous glass to the moulds, can be avoided by using coatings similar to those which are proposed for glassmaking moulds.
Thus, document U.S. Pat. No. 3,994,703 teaches the protection of troughs for guiding a charge or parison of glass towards a forming station, by means of a coating composed of a lubricating pigment, graphite or molybdenum bisulphide, with a thermosettable polymer as binder, resistance to wear or abrasion being augmented by the presence, between the substrate and the coating, of a porous supporting layer consisting of a transition metal with a transition metal carbide or nitride.